A couple of good horrors that I read recently that I really liked were Scott Sigler's "Contagious" and "Infected". Not really post-apocalyptic, but the world is teetering on the brink of it if things don't work out right.

If you've never tried Richard Laymon, I highly recommend him if you've got a taste for gut-churning horror, which apparently you do if you like Keene. I'd recommend "In the Dark" as a starter just because it's the first of his that I read and it made me want more from him.

All the above are available for your Kindle, and the Laymon is pretty cheap ($6.39).

I'm sure you'll get plenty of other good recommendations, and here's a few threads worth looking into: